PHYS112 : Labs

C. Oscilloscope Guide

Introduction

An oscilloscope is a device used to observe and measure time-dependent electronic signals. It is essentially an enhanced voltmeter which displays a graph of potential difference vs. time. It can also draw graphs of one voltage signal vs. another. Both the vertical voltage scale and the horizontal time scales of the oscilloscope display are adjustable so that you can look at signals as small as 10 mV or as large as 50 V over time scales as short as 1 $\mu s$ and as long as 1 s.

An oscilloscope can draw you a snapshot of your signal over a time period that you specify. Much like when you have collected potential vs. time graphs with the LabPro interface, you will specify the time scale of each measurement. Unlike the LabPro interface, the oscilloscope collects continuous (analog) data instead of collecting discrete data points at a specific sampling rate, so you will not need to set a sampling rate.

Instead of collecting a single snapshot of the input signal, an oscilloscope repeatedly updates the display with newly collected snapshots. Each snapshot is called a trace. This approach has an inherent problem. If the oscilloscope were to simply display one trace immediately after another, an unsynchronized jumble of traces would result. An illustration of the problem is shown in Figure 32 in which the signal of interest oscillates sinusoidally.

**Figure 32:** Multiple unsynchronized snapshots of the same sinusoidal signal.
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Oscilloscopes address this problem by updating the display based on a trigger. Traces are drawn starting at a specified voltage level and with a specified slope condition (rising or falling). The triggering level and slope conditions are illustrated in Figure 33 for the same sinusoidal signal used in Figure 32. With triggering, each trace overlaps the last, and a stable display is obtained.

**Figure 33:** Two oscilloscope displays of the same signal with the same triggering level, (a) triggering on a positive slope and (b) triggering on a negative slope.
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Controls

Below is a description of the control panels of each of the two models of Leader oscilloscopes we have in the laboratory. Look for the model number of your oscilloscope on its front panel. Images of the control panels of the model 1020 and 1021 oscilloscopes are shown in Figures 34 and 35, respectively. Knobs and switches are labeled for reference in the description below.

**Figure 34:** Control panel of the Leader 1020 20 MHz oscilloscope.
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**Figure 35:** Control panel of the Leader 1021 20 MHz oscilloscope.
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Vertical Controls

A. Signal Input
This is a coaxial connector. You need either a coaxial-to-alligator cable or an oscilloscope probe to connect your circuit to the oscilloscope.
B. Vertical Display Mode
This switch determines which signal or combination of signals is displayed. The CH1 and CH2 settings display channels 1 and 2, respectively. The ALT setting displays both signals by alternating between drawing full traces of the two channels. The CHOP setting alternates between the two channels during each sweep across the display. At low frequencies, the CHOP setting is easier on the eyes. The ADD setting displays the sum of the signals on channels 1 and 2.
C. Input Signal Coupling
Each input channel has its own signal coupling switch. The DC setting couples the entire signal to the oscilloscope. The AC setting removes any constant (DC) component of the input signal.
The GND setting grounds the input, which gives a flat line at 0 V on the graph.
D. Vertical Scale / Variable
Each input channel has an independent vertical scale knob. The outer knob sets the VOLTS/DIV for the corresponding channel. This is the vertical scale of the displayed graph in units of V or mV per division (DIV) as indicated by the labels surrounding the knob. One division (DIV) corresponds to the height of one of the squares of the display grid.
The inner knob is used to vary the vertical scale continuously. Use with caution! If this knob is not turned completely to the right (clockwise), the VOLTS/DIV setting of the outer knob is not accurate. Always check the inner knob before making voltage measurements.
E. Vertical Position
Each input channel has an independent position knob which shifts the vertical position of the graph of the signal on the display.

Horizontal Controls

F. Horizontal Position
The horizontal position knob shifts the horizontal position of the graph(s) on the display.
G. Horizontal Scale
The horizontal scale knob sets the TIME/DIV for all signals on the display. This gives the horizontal scale of the displayed graph(s) in units of s, ms, or $\mu s$ per division (DIV) as indicated by the labels surrounding the knob. One division (DIV) corresponds to the width of one of the squares of the display grid.
The X-Y setting of knob G puts the oscilloscope in a mode in which it graphs the signal on input channel 2 vs. the signal on input channel 1. In X-Y mode, the horizontal scale is set by the voltage scale knob (knob D) of channel 1 instead of by knob G, and the horizontal position is controlled by the vertical position knob (knob E) of channel 1 instead of by knob F.
H. Variable Scale
The variable knob is used to vary the horizontal scale continuously. Use with caution! If this knob is not turned completely to the right (clockwise), the TIME/DIV setting is not accurate. Always check the inner knob before making time measurements.

Triggering Controls

I/J. Level, Slope, and Mode
The level, slope, and mode settings are handled differently by the model 1020 and 1021 oscilloscopes.
Model 1020
The outer part of knob I controls the triggering level. The triggering slope is set by switch J. These settings are described above in the Introduction.
The triggering mode can be set to either NORM or AUTO by pulling knob I out or pushing it in, respectively. In the normal mode (NORM), the oscilloscope only draws a graph if it can satisfy the level and slope conditions. In the AUTO mode, it will show a signal even if it can't trigger. The AUTO mode is helpful if you are having a hard time finding a signal.
The inner part of knob I controls HOLDOFF. This is for triggering on complex digital signals. You will not need to use it. It should be left in the NORM position (fully counterclockwise).
Model 1021
Knob I controls the triggering level. The triggering slope is positive if knob I is pushed in and negative if knob I is pulled out. The level and slope settings are described above in the Introduction.
The triggering mode is set by switch J. In the normal mode (NORM), the oscilloscope only draws a graph if it can satisfy the level and slope conditions. In the AUTO mode, it will show a signal even if it can't trigger. The AUTO mode is helpful if you are having a hard time finding a signal.
The knob labeled HOLDOFF is for triggering on complex digital signals. You will not need to use it. It should be left in the NORM position (fully counterclockwise).
K. Trigger Signal Source
This switch determines which signal triggers oscilloscope to draw graphs on the display. The CH1 and CH2 settings display channels 1 and 2, respectively. The LINE setting triggers on the AC power signal supplied to the oscilloscope. The ALT setting triggers each input channel independently on itself. (Model 1021 does not have an ALT setting.)
L. Trigger Signal Coupling
This switch determines how the trigger signal is coupled. The DC setting couples the entire trigger signal to the oscilloscope. The AC setting removes any constant (DC) component of the trigger signal.
The HF-REJ and LF-REJ settings are used to reject high-frequency and low-frequency components of the trigger signal, respectively (with low-pass and high-pass filters). They help to reduce false triggering when you are looking at either the low-frequency or high-frequency component of a complex signal. (Model 1020 does not have a LF-REJ setting.)